Xylanase treatment of chemical pulp

ABSTRACT

The present invention discloses methods of bleaching chemical pulp that use xylanase enzymes after chemical bleaching. The method comprises the steps of carrying out a chlorine dioxide stage to produce a partially bleached pulp, treating the partially bleached pulp with a xylanase enzyme, optionally in the presence of oxygen and hydrogen peroxide, in a mild extraction stage, then bleaching the pulp with a second chlorine dioxide stage. The method allows the mill to decrease the usage of sodium hydroxide or other alkali, while decreasing the use of chlorine dioxide, and possibly improving the yield and strength of the pulp, while maintaining a similar level of bleached brightness of the pulp. The pulp bleaching method of the present invention may be performed in a pulp mill as part of a complex pulp bleaching process.

This application is the U.S. National Stage of International ApplicationNo. PCT/CA03/00300, filed Mar. 5, 2003, published in English, whichclaims the benefit of U.S. provisional application No. 60/362,200, filedMar. 6, 2002.

The invention relates to methods of bleaching pulp. More specificallythe invention relates to methods of bleaching pulp using xylanaseenzymes.

BACKGROUND OF THE INVENTION

The production of bleached chemical pulp is a major industry around theworld. More than 50 million tons of bleached pulp is produced annually.Bleached chemical pulp is the largest component of all types of whitepaper, including that used in photocopy paper, writing paper, and paperpackaging. In addition, bleached chemical pulp is also used to impartstrength to less expensive grades of paper, such as newsprint. Bleachedchemical pulp has large markets because of its high degree of whitenessand cleanliness, the stability of the whiteness, its high strength, andthe ease and uniformity of the printing surface it provides. Theseattributes are obtained when lignin, which is colored and decreases theinterfiber bonding of the cellulose, is almost completely removed fromthe pulp.

In the process of chemical pulping, the furnish (or feedstock) primarilyconsists of wood chips which are added to a reaction chamber, known as adigester, and are treated with chemicals to dissolve lignin in the pulp.There are several chemical pulping processes known in the art. Two ofthe major chemical pulping processes are kraft pulping, in which thepulp is cooked in alkaline liquor, and sulfite pulping, in which thepulp is cooked in acidic liquor. Both kraft pulping and sulfite pulpingmay be performed in batch or continuous digestors.

One of the main purposes of the pulping process is to release ligninwhich binds cellulose fibers in the feedstock. Pulping dissolves 85% to95% of the lignin in the feedstock material. Following the pulpingstage, the pulp is washed with water to remove dissolved lignin.

While pulping removes most of the lignin in the feedstock material, itis not capable of removing all the lignin without destroying thecellulose fibers of the feedstock. The remaining lignin is removed fromthe pulp by bleaching.

A pulp bleaching process may consist of many stages. For example,following pulping, a pulp bleaching process may comprise an alkalineoxygen delignification stage (O), an enzymatic treatment stage (X), oneor more chlorine dioxide stages (D), and one or more alkaline extractionstages (E). A pulp bleaching process may also comprise one or more waterwashes or alternatively, each stage may comprise a water wash as a finalstep of the stage. Thus, a representative pulp bleaching sequence inwhich pulp is bleached using three chlorine dioxide stages and twoalkaline extraction stages may be represented as D-E-D-E-D. Similarly, apulp bleaching sequence wherein pulp is subjected to an alkaline oxygendelignification stage, an enzymatic treatment stage, three chlorinedioxide bleaching stages and two alkaline extraction stages wherein eachstage is followed by a water wash may be represented by O-X-D-E-D-E-D.

It is common for mills to perform an alkali-oxygen delignification stageprior to carrying out chemical bleaching of pulp. This process consistsof reacting the pulp with oxygen and alkali at high temperatures(approximately 100° C.) for a period of about one hour. Alkali-oxygendelignification reduces the amount of lignin in the pulp by 35-50%, butthis process is harsh on the pulp and is often accompanied bydestruction of some of the cellulose fibers in the pulp. Followingalkali-oxygen delignification, the pulp is washed as described earlierto remove solubilized lignin.

The next bleaching stage after alkali-oxygen delignification is usuallychemical bleaching with oxidative chemicals, the most prominent beingchlorine dioxide (ClO₂). However, several processes have been describedwhich may facilitate or enhance bleaching of pulp prior to chemicalbleaching. For example, an enzymatic treatment stage with xylanase maybe used to enhance the bleaching of pulp prior to chemical bleaching.

Xylanases are used in the pulp and paper industry to enhance thebleaching of pulp and to decrease the amount of chlorinated chemicalsused in bleaching stages (Erickson, 1990; Paice et al., 1988; Pommier etal., 1989). There have been several mechanisms proposed for thebleaching action of xylanase. One is that lignin is connected tocrystalline cellulose through xylan and xylanase enzymes facilitatebleaching of pulp by hydrolysing xylan, releasing coloured lignin fromthe pulp. A second proposed mechanism is that xylanase removes xylanthereby improving the alkali extractability of the pulp. Regardless ofthe mechanism, xylanase treatment allows subsequent bleaching chemicalssuch as chlorine, chlorine dioxide, hydrogen peroxide, or combinationsof these chemicals to bleach pulp more efficiently than in the absenceof xylanase. Pretreatment of pulp with xylanase prior to chemicalbleaching increases the whiteness and quality of the final paper productand reduces the amount of chlorine-based chemicals which must be used tobleach the pulp. This in turn decreases the chlorinated effluentproduced by such processes.

Xylanases have been isolated from a variety of organisms includingbacteria and fungi. Generally, fungal xylanases exhibit optimal activityat acidic pHs, in the range of about 3.5 to 5.5, and a temperature ofabout 50° C. In contrast, bacterial xylanases exhibit optimal activityat pH 5 to pH 7 and a temperature optimum between 50° C. and 70° C.However, there are other xylanase enzymes which exhibit optimal activityunder other conditions. For example, U.S. Pat. No. 5,405,789 to Campbellet al., discloses construction of thermostable mutants of low molecularmass xylanase from Bacillus circulans. U.S. Pat. No. 5,759,840 to Sunget al., discloses modification of a family 11 xylanase from Trichodermareesei to improve thermophilicity, alkalophilicity and thermostabilityas compared to the natural xylanase. U.S. Pat. No. 5,916,795 to Fukunagaet al., discloses a thermostable xylanase from Bacillus. A publicationentitled “Xylanase Treatment of Oxygen-Bleached Hardwood Kraft Pulp atHigh Temperature and Alkaline pH Levels Gives Substantial Savings inBleaching Chemicals” to Shah et al., (J. of Pulp and Paper Science, vol26 No. 1 January 2000, which is herein incorporated by reference)discloses treating oxygen delignified hardwood pulp with xylanase fromThermotoga maritima at pH 10 and 90° C. and subsequently bleaching thepulp. These documents disclose using xylanases to enzymatically treatpulp prior to chemical bleaching. However, none of these documentssuggest using xylanases to treat pulp after a chemical bleaching stage.

The next stage in a typical pulp bleaching process is usually chlorinedioxide bleaching with chlorine dioxide, chlorine or in some instances,a combination of chlorine dioxide and other oxidative bleaching agents.For example, the first chlorine dioxide stage in a chemical bleachingprocess is often called the D_(o) or D100 stage. Subsequent chlorinedioxide bleaching stages are referred to as D₁, D₂ and so on. For millsthat bleach pulp without an alkali-oxygen delignification stage, theD_(o) stage is the first chemical bleaching stage. The D_(o) stage isusually carried out at pH 1.5 to 3.0. In a small but decreasing numberof mills, up to 30% to 50% chlorine gas may be added to ClO₂ in aneffort to achieve a higher efficiency of lignin removal. Such a stage isreferred to as a C_(D) stage. After a D_(o) or C_(D) stage, the pulp iswashed with water, and alkaline extracted. Alkaline extraction iscarried out by adjusting the pH of the pulp to 9.0 to 12.0 with sodiumhydroxide or sodium carbonate at a temperature between 60° C. to 120° C.for a period of 30 to 90 minutes. After the alkaline extractions stage,the pulp is washed with water. The sequence of chlorine dioxidebleaching stage, wash and alkaline extraction is repeated until the pulpis suitably bleached. In most cases, two to three rounds of acidic andalkaline bleaching, alternating between chlorine dioxide stages andalkaline extraction stages, are required before the pulp is suitablybleached.

In all present commercial applications, xylanase use comprises axylanase treatment stage prior to the first chlorine dioxide stage. Thisresults in a pulp with increased brightness compared to pulp treated ina similar manner but without xylanase treatment. Alternatively, aspecific brightness level can be achieved using a smaller amount ofbleaching chemicals when the pulp is treated with xylanase prior tobleaching, compared to pulp that is not treated with xylanase beforebleaching.

U.S. Pat. No. 5,645,686 discloses a process for bleaching a chemicalpaper pulp by means of a sequence of treatment stages involving at leastone stage with hydrogen peroxide and at least one stage with aperoxyacid. The patent also discloses a xylanase treatment stage incombination with the pulp bleaching sequence. The patent does notsuggest subjecting pulp to a xylanase treatment stage after a chlorinedioxide stage in a pulp bleaching process which employs only chlorinedioxide bleaching stages. Further, there is no teaching as to whether axylanase treatment stage after a first chlorine dioxide bleaching stagemay be more effective in enhancing the bleaching of pulp compared to apulp bleaching sequence wherein xylanase treatment is performed prior tothe first chlorine dioxide bleaching stage.

WO 91/05908 discloses a process for producing bleached lignocellulosicpulp having reduced organically bound chlorine and reduced brightnessreversion. The process entails treating pulp with xylanase after achemical bleaching stage which primarily employs chlorine. The referenceteaches that xylanase treatment after a chlorine bleaching stage is notas effective at bleaching pulp as xylanase treatment prior to a chlorinebleaching stage. The reference does not address whether a chlorinedioxide bleaching stage, as employed now by most mills, followed by axylanase treatment stage may be capable of enhancing the bleaching ofpulp.

A publication entitled Xylanase Pre- and Post-treatments of BleachedPulps Decrease Absorption Coefficient by Wong et al., (2000. J. of Pulpand Paper Science Vol 26 No. 10 377-383, which is herein incorporated byreference) teaches xylanase treatment of pulp as a final stage of apartial or complete chemical bleaching process. However, the referenceteaches that xylanase treatment of pulp after chemical bleachingincreases the brightness of pulp by a smaller amount than doesconventional xylanase treatment of pulp before chemical bleaching.

While the xylanase treatments in pulp bleaching processes generallyresult in enhanced pulp bleaching compared to equivalent pulp bleachingprocesses which do not comprise xylanase treatment, there is a need inthe art to increase the efficiency of the xylanase treatment. The pulpindustry is under pressure to decrease the use of chlorine-containingbleaching chemicals, such as chlorine and chlorine dioxide, and thus,any method or process which can be integrated into a pulp bleachingprocess to reduce the use of chlorine-containing bleaching chemicals orthe toxic effluents produced by the use of such chemicals would be animportant and valuable asset to the pulp industry. The industry wouldalso save money by using less chemicals such as, chlorine dioxide inbleaching stages, and sodium hydroxide and hydrogen peroxide in alkalineextraction stages. Improving the efficiency of xylanase treatment wouldaddress these concerns by further decreasing chemical usage.

There is a need in the art for novel methods and more efficient methodsof bleaching pulp. Further, there is a need in the art for methods, orprocesses which can be integrated into existing pulp bleaching processesto increase the efficiency of the bleaching process and reduce the useof chlorine containing bleaching compounds or the toxic effluentsproduced by the use of such chemicals. There is also a need to savemoney by decreasing chemical usage.

It is an object of the invention to overcome drawbacks in the prior art.

The above object is met by a combination of the features of the mainclaims. The sub claims disclose further advantageous embodiments of theinvention.

SUMMARY OF THE INVENTION

The invention relates to methods of bleaching pulp. More specificallythe invention relates to methods of bleaching pulp using xylanase.

According to the present invention, there is provided a method ofbleaching chemical pulp comprising the steps of:

-   -   a) exposing chemical pulp to a chlorine dioxide bleaching stage        to produce a partially bleached pulp;    -   b) treating the partially bleached pulp with a xylanase in a        mild extraction stage, at a pH of about 3 to 8, more preferably        about 5 to 8; and    -   c) exposing the enzymatically treated pulp to a second chlorine        dioxide bleaching stage without an alkaline extraction stage        between the first chlorine dioxide bleaching stage and the        second chlorine dioxide bleaching stage.

The method of the invention replaces the conventional alkalineextraction stage, which typically takes place at pH 10.0-12.5 at 70° C.to 120° C., with a much milder extraction stage. In the presence ofxylanase, the extraction stage takes place at near neutral to acid pH,at temperatures as low as 60° C. By using a mild extraction stage, amill may decrease the use of sodium hydroxide, or other alkali used toachieve high pH. The mill also decreases the use of bleaching chemicalssuch as chlorine dioxide. The mill also avoids the harsh extractionconditions that can destroy or degrade the cellulose fibers, thuspermitting production of a pulp with a higher strength and/or a higheryield than conventionally produced pulps.

The pulp bleaching method may be performed in a mill as part of a largerpulp bleaching process. Further, the chemical pulp may comprise kraftpulp, soda pulp or sulfite pulp.

Also according to the method of the present invention as defined above,the first bleaching step may be preceded by an alkaline oxygendelignification stage.

Any xylanase active at conditions of the mild extraction stage may beemployed in the method of the present invention. For example, but not tobe considered limiting, the xylanase may be selected from the groupconsisting of wild-type Trichoderma reesei xylanase II, TrX-HML-AHAE,TrX-HML-GHAE, TrX-HML-ARAE, TrX-HML-GRAE, TrX-HML-GPHAE, andTrX-HML-GPRAE.

According to a further aspect of the present invention there is provideda method of bleaching chemical pulp comprising the steps of:

-   -   a) treating chemical pulp with a first xylanase in a first        enzyme treatment stage to produce an enzyme treated pulp;    -   b) exposing the enzyme treated pulp to a first chlorine dioxide        bleaching stage to produce a partially bleached pulp; and    -   c) treating the partially bleached pulp with a second xylanase        in a mild extraction stage at a pH of about 3 to 8, preferably        about 5 to 8; and    -   d) exposing the enzymatically treated pulp to a second chlorine        dioxide bleaching stage without an alkaline extraction stage        between the first chlorine dioxide bleaching stage and the        second chlorine dioxide bleaching stage.

Also according to the method of the present invention as defined above,the first step of treating the chemical pulp with a first xylanase in afirst enzyme treatment stage may be preceded by an alkaline oxygendelignification stage.

Further, according to the present invention as defined above, the firstxylanase may be different from the second xylanase or the first xylanasemay be identical to the second xylanase. The first xylanase or thesecond xylanase may be selected from the group consisting ofTrX-HML-AHAE xylanase which is commercially available from IogenCorporation, and wild-type Trichoderma reesei xylanase II, or othersuitable xylanase. Further, the conditions of the first enzyme treatmentstage may be different from the conditions of the second enzymetreatment stage or the conditions of the first enzyme treatment stagemay be identical to conditions in the second enzyme treatment stage.

Also according to the method of the present invention as defined above,the method may be followed by one or more enzyme treatment stages,chemical bleaching stages, water washes, and extraction stages. Thus,the pulp bleaching method of the present invention may form part of amore elaborate or complex pulp bleaching process.

The pulp bleaching method of the present invention enhances pulpbleaching compared to conventional pulp bleaching processes known in theart. Further, the pulp bleaching method of the present invention iseasily integrated into pulp bleaching processes which are currentlypractised in the art.

The chlorine dioxide bleaching stages used within the method asdescribed above, may comprise chlorine dioxide or chlorine dioxide andat least one other bleaching agent selected from the group consisting ofchlorine, ozone, hydrogen peroxide or a combination thereof, such thatchlorine dioxide is the more abundant bleaching agent in the chlorinedioxide bleaching stage. Chlorine dioxide bleaching may be performed ata pH between about 1 and about 5, but preferably about 1.5 to about 3.

Also, according to the method of the present invention as defined above,the step of treating chemical pulp with a xylanase in an enzymetreatment stage may be preceded by an alkaline oxygen delignificationstage.

Further, according to the present invention, there is provided a pulpbleaching process comprising a sequence selected from the groupconsisting of:

-   -   a) Do-X-D-E-D;    -   b) Do-X-D-X-D;    -   c) Do-X-D-D;    -   d) O-Do-X-D-E-D;    -   e) O-Do-X-D-D;    -   f) Do-Xop-D-E-D;    -   g) X-Do-Xop-D-E-D; and    -   h) X-Do-X-D-E-D        wherein,

-   Do is a chlorine dioxide bleaching stage,

-   X is a mild extraction xylanase treatment stage,

-   D is a chlorine dioxide bleaching stage,

-   E is an alkali extraction stage, and

-   Xop is a mild extraction stage comprising xylanase, oxygen and    hydrogen peroxide.

This summary does not necessarily describe all necessary features of theinvention but that the invention may also reside in a sub-combination ofthe described features.

DESCRIPTION OF PREFERRED EMBODIMENT

The invention relates to methods of bleaching pulp. More specificallythe invention relates to methods of bleaching pulp using xylanase.

The following description is of a preferred embodiment by way of exampleonly and without limitation to the combination of features necessary forcarrying the invention into effect.

In one embodiment of the present invention, there is provided a methodof bleaching chemical pulp comprising the steps of:

-   -   a) exposing chemical pulp to a chlorine dioxide bleaching stage        to produce a partially bleached pulp;    -   b) treating the partially bleached pulp with a xylanase in a        mild extraction stage at pH about 3 to 8, preferably about 5 to        8, to produce an enzymatically treated pulp; and    -   c) exposing the enzymatically treated pulp to a second chlorine        dioxide bleaching stage without an alkaline extraction stage        between the first chlorine dioxide bleaching stage and the        second chlorine dioxide bleaching stage.

It is preferred that the first chlorine dioxide bleaching stagecomprises a water wash as a final step of the stage prior to the step oftreating the partially bleached pulp with a xylanase in the mildextraction stage. More preferably, both chlorine dioxide bleachingstages and the mild extraction stage comprise a water wash as a finalstep of each stage. Further, the pulp bleaching method of the presentinvention may be performed in a mill as part of a complex pulp bleachingprocess.

The present invention also pertains to a method of bleaching chemicalpulp as outlined above, wherein some of the bleaching stages compriseperoxyacid.

By the term “chemical pulp” it is meant any type of virgin fiber,secondary fiber, woody or nonwoody fiber, softwood, hardwood or amixture thereof which has been treated by chemical pulping such as, butnot limited to, kraft pulp, soda pulp or sulfite pulp and issubsequently in a form suitable for bleaching. Preferably, the chemicalpulp comprises virgin fiber. Chemical pulp also includes kraft pulp,soda pulp or sulfite pulp which has been exposed to an alkali oxygendelignification stage prior to practising the method of the presentinvention. The alkaline oxygen delignification stage is preferablyfollowed with a water wash prior to the first chlorine dioxide stage.Other conditions associated with the production of chemical pulp,including kraft and sulfite pulps are described in “Pulp Bleaching:Principles and Practice” (edited by Dence and Reeve, 1996; which isherein incorporated by reference).

By the term “chlorine dioxide bleaching stage” it is meant treating pulpwith chlorine dioxide, or chlorine dioxide in combination with chlorine,ozone, hydrogen peroxide, both chlorine and ozone or chlorine andhydrogen peroxide. Preferably, chlorine dioxide is the more abundantbleaching agent in the chlorine dioxide bleaching stage. Other aspectsof chlorine dioxide bleaching which may be used in the method of thepresent invention are described in Dence and Reeve (1996, “PulpBleaching: Principles and Practice”).

In a preferred embodiment, the first chlorine dioxide bleaching stage isperformed at a pH in the range of about 1 to about 5, preferably about1.5 to about 3. The temperature of the stage is preferably 50° C. to 75°C., and the reaction time is 5 minutes to 60 minutes. The amount ofchlorine dioxide added to the pulp is 5 to 25 kilograms per tonne ofpulp. These conditions are similar to the first chlorine dioxidebleaching stage in a pulp mill, as would be known to one of skill in theart.

The mild extraction stage is run at a pH of about 3 to 8. In this range,a significantly lower amount of sodium hydroxide or other alkali isrequired to adjust the pH of the pulp, as compared with a conventionalalkaline extraction at pH 10 to 12.5, and from 70° C. to 120° C.Preferably, the pH of the mild extraction stage is about pH 5 to 8 andmost preferably pH 7 to 8. The pH is measured at the end of the stage,at the end of the tower or in the washer vat. The pulp consistency ispreferably 5% to 10%.

The mild extraction stage optionally includes hydrogen peroxide, oxygen,or a combination of these compounds that are often found in aconventional alkaline extraction stage. When oxygen is used, it ispreferably added at a level corresponding to 3 to 9 kg per tonne pulp.Hydrogen peroxide is preferably used at a level of 2 to 7 kg per tonnepulp. Performance additives such as magnesium sulfate, that are oftenused in conventional alkaline extraction stages, may also be used inmild extraction stages.

The mild extraction stage is preferably run at a temperature of 50° C.to 80° C. Any xylanase which is capable of hydrolyzing xylan andenhancing the bleaching of pulp under the conditions of the mildextraction stage may be used in the method of the present invention. Thexylanase must be active at the pH and temperature of the stage, and beresistant to oxygen, hydrogen peroxide, and additives that may bepresent.

The xylanase dosage is preferably 0.5 to 2.0 xylanase units per gram ofpulp. Methods of measuring xylanase units are described in Example 2.

Both wild-type and genetically modified xylanases may be employed in themethod of the present invention. For example, but not wishing to belimiting, xylanases which may be useful in the method of the presentinvention include fungal xylanases which exhibit optimal activity atacidic pHs in the range of about 3.5 to about 5.5 and at temperatures ofabout 50° C., and bacterial xylanases which exhibit optimal activity atpH 5 to 7 and temperatures between about 50° C. and 70° C. Also, thepresent invention contemplates using other xylanase enzymes under otherconditions such as, but not limited to wild-type, thermostable andalkalostable xylanases as taught in U.S. Pat. No. 5,405,789 whichdiscloses mutants of low molecular mass from Bacillus circulans, andU.S. Ser. No. 60/213,803 to Sung (which is herein incorporated byreference), which discloses xylanases having increased thermophilicityand alkalophilicity relative to the wild-type Trichoderma xylanase, orwild-type thermophilic enzyme. Further, other xylanases which may beuseful in the method of the present invention include thermostablexylanases such as Caldocellum saccharolyticum, Thermotoga maritima andThermotoga sp. Strain FJSS-B.1 (Lüthi et al. 1990; Winterhalter et al.1995; Simpson et al. 1991; which are herein incorporated by reference).The method of the present invention further contemplates the use ofxylanases derived from, but not limited to Trichoderma reesei xylanaseI, Trichoderma viride xylanase, Streptomyces lividans xylanase B,Streptomyces lividans xylanase C, or other non-family 11 xylanases, forexample, but not wishing to be limiting, Caldocellum saccharolyticum,Thermotoga maritima and Thermotoga sp. Strain FJSS-B.1.

Genetically modified variants of these xylanases also may be used incombination or alone in the enzyme treatment stages of the presentinvention provided they are capable of enhancing the bleaching of pulp,that is enhancing removal of lignin from pulp under the conditions ofthe mild extraction stage. Genetically modified variants might havesuperior ability to act over a wider range of pH, temperature, orconcentration of oxygen or hydrogen peroxide than the correspondingwild-type xylanase.

As is evident to someone of skill in the art, some native xylanasesexhibit both xylanase and cellulase activities. The additionalcellulolytic activity is undesirable for pulp bleaching due to itsdetrimental effect on cellulose, the bulk material in pulp fibres. It ispreferred that the method of the present invention use one or morexylanases which lacks cellulolytic activity or is reduced incellulolytic activity. Preferably, the method of the present inventionuses one or more xylanases that have reduced or impaired cellulaseactivity.

After the mild extraction stage, the amount of lignin associated withpulp may be estimated by determining the kappa number of the pulp, whichmay be performed according to Example 1. A method, process or step whichreduces the kappa number of the pulp by a greater amount than anothermethod, process, or step may be considered to be more effective inremoving lignin associated with pulp and thus, is more effective inenhancing the bleaching of pulp.

The second chlorine dioxide stage is carried out using proceduresfamiliar to those skilled in the art, and described in Dence and Reeve.Preferably, this stage is run at a pH of 3 to 6, a temperature of 60° C.to 90° C., and a time of 1 to 4 hours. The use of a mild extractionstage may decrease the amount of acid, if required to adjust the pH inthe second chlorine dioxide stage.

In practicing the present invention there is no alkaline extractionstage between the first and second chlorine dioxide stages. An alkalineextraction stage is well known in the art to take place at pH 8.5 to 13,most often pH 9 to 11, at 60° C. to 90° C. or up to 120° C. in “hot”alkaline extractions. The mild extractions of the present inventionoccur in a lower pH range than a conventional alkaline extraction.

After the second chlorine dioxide stage, the brightness of the pulp maybe determined according to Example 7. A method, process or step thatproduces a pulp with a higher ISO brightness number than another method,process or step is more effective in enhancing the bleaching of pulp.

According to the present invention, there is provided a method ofbleaching chemical pulp with xylanase. In an aspect of an embodiment ofthe present invention, there is provided a method of bleaching chemicalpulp comprising the steps of:

-   -   a) treating the chemical pulp with a first xylanase in a first        enzyme treatment stage to produce an enzyme treated pulp,    -   b) exposing the enzyme treated pulp to a chlorine dioxide        bleaching stage to produce a partially bleached pulp; and    -   c) treating the partially bleached pulp with a second xylanase        in a second enzyme treatment stage at a pH of about 3 to 8,        preferably about 5 to 8; and    -   d) exposing the enzymatically treated pulp to a second chlorine        dioxide bleaching stage without an alkaline extraction stage        between the first chlorine dioxide bleaching stage and the        second chlorine dioxide bleaching stage.

The first xylanase treatment stage may be preceded by an alkaline oxygendelignification stage.

The first xylanase employed in the first enzyme treatment stage may beidentical to the second xylanase used in the second enzyme treatmentstage, or the first xylanase may be different from the second xylanase.Further, the conditions of the first enzyme treatment stage may beidentical or dissimilar to the conditions in the second enzyme treatmentstage. Conditions of the enzyme treatment stage include, but are notlimited to temperature, pH, incubation time, amount of xylanase used,components of the incubation medium, and pulp consistency. As would beevident to someone of skill in the art, it is preferred that theconditions of an enzyme treatment stage are compatible with the xylanaseenzyme or enzymes used in that enzyme treatment stage. Specifically, theconditions of each enzyme treatment stage should allow the xylanase usedin the enzyme treatment stage to exhibit more than about 10% of itsmaximum activity, and more preferably greater than about 30% of itsmaximum activity under the conditions of the enzyme treatment stage.Thus, it may be possible that an extremely alkalophilic xylanase used inthe first enzyme treatment stage may exhibit less than 10% of itsmaximum activity under the conditions of the second enzyme treatmentstage, that is under pH conditions between 3 and 8. It is preferred thatsuch an alkalophilic xylanase is not used in the second enzyme treatmentstage. The activity of a xylanase may be determined by any method knownin the art, for example, but not limited to the assays described inExample 2.

Without wishing to be limiting, the first xylanase, second xylanase orboth xylanases may comprise the wild-type Trichoderma reesei xylanase ora genetically modified variant thereof such as, but not limited to,

-   -   TrX-HML-AHAE*,    -   TrX-HML-GHAE*,    -   TrX-HML-ARAE*,    -   TrX-HML-GRAE*,    -   TrX-HML-GPHAE*, or    -   TrX-HML-GPRAE*,        (* disclosed in WO 01/92487; which is incorporated herein by        reference), or other modified xylanases disclosed in WO 01/92487        that exhibit properties of a xylanase as defined above. The        xylanase may also comprise Actinomadura flexuosa xylanase A as        described in U.S. Pat. No. 5,935,836 (herein incorporated by        reference) as a 35 kDa xylanase.

The pulp bleaching method of the present invention enhances pulpbleaching compared to conventional pulp bleaching processes known in theart. Further, the pulp bleaching method of the present invention iseasily integrated into pulp bleaching processes which are currentlypractised in the art.

Representative pulp bleaching sequences contemplated by the presentinvention are described in FIG. 1. The pulp bleaching sequences are forillustrative purposes only and are not meant to limit the invention inany manner. The method of the present invention contemplates pulpbleaching sequences comprising a chlorine dioxide stage, followed by amild extraction stage with xylanase treatment, followed by a chlorinedioxide bleaching stage (Do-X-D) without an alkaline extraction stageintervening between the two chlorine dioxide stages. This is representedby bleaching sequences such as, but not limited to:

-   -   Do-X-D-E-D,    -   Do-X-D-X-D,    -   O-Do-X-D-E-D,    -   O-Do-X-D-X-D,    -   Do-X-D-D,    -   O-Do-X-D-D,    -   Do-Xop-D-E-D,    -   X-Do-Xop-D-E-D, and    -   X-Do-X-D-E-D,        wherein,

-   Do is a chlorine dioxide bleaching stage,

-   X is a mild extraction xylanase treatment stage,

-   D is a chlorine dioxide bleaching stage,

-   E is an alkali extraction stage, and

-   Xop is a mild extraction stage comprising xylanase, oxygen and    hydrogen peroxide.

Further, a pulp bleaching sequence comprising two or more stages denotedby the same character may be performed under identical or dissimilarconditions. For example, but not wishing to be limiting, a pulpbleaching sequence comprising three D stages may comprise identical ordifferent treatment conditions in each stage. In the case of a xylanasetreatment stage which is practised after a chlorine dioxide bleachingstage in a pulp bleaching sequence, such as, but not limited toDo-X-D-E-D and Do-X-D-X-D pulp bleaching sequences, the xylanasetreatment stage may replace an alkaline extraction stage. In such anembodiment, replacing an alkaline extraction stage with a xylanase.treatment stage may reduce the usage of base, such as, but not limitedto sodium hydroxide for pH adjustment of the pulp.

It is also contemplated by the method of the present invention that anenzyme treatment stage comprising xylanase, which is performed after achlorine dioxide bleaching stage may replace an alkaline extractionstage. For example, but not wishing to be limiting in any manner, a pulpbleaching sequence of the present invention, such as, but not limited toDo-X-E-D-E-D may be modified to Do-X-D-E-D or Do-X-D-X-D. These pulpbleaching sequences, and others that describe similar bleachingsequences are contemplated by the method of the present invention. Inall of the pulp bleaching sequences described above, a mild extractionstage, which is performed after a chlorine dioxide bleaching stage, isperformed at a pH of about 3 to 8.

Referring now to Table 2 (Example 8), there is shown the effect oftreating pulp with a mild extraction stage comprising xylanase followinga chlorine dioxide bleaching stage as contemplated in the pulp bleachingsequences of the method of the present invention. Specific details ofthe pulp bleaching sequences are described in Example 8.

The results in Table 2 show that a xylanase untreated control pulpbleaching sequence (Do-E) requires a sodium hydroxide charge of about1.2% (w/w) relative to the mass of the pulp to adjust the pH of the pulpto about 11.2 in an alkaline extraction stage following a chlorinedioxide bleaching stage. The pulp bleaching sequence (Do-E) produces apulp having a kappa number of about 6.3. Treating pulp with a xylanaseenxyme, for example but not limited to,

i) TrX-HML-AHAE xylanase at a pH of about 6.8 (Do-X) after a chlorinedioxide bleaching stage requires a sodium hydroxide charge of about 0.2%(w/w) relative to the mass of the pulp. The pulp bleaching sequenceproduces a pulp having a kappa number of about 5.4.

ii) HTX-18 xylanase at a pH of about 7.2 (also Do-X) after a chlorinedioxide bleaching stage requires a sodium hydroxide charge of about 0.2%(w/w) relative to the mass of the pulp. The pulp bleaching sequenceproduces a pulp having a kappa number of about 6.1.

iii) ECOPULP TX-1200C xylanase at a pH of about 6.8 (also Do-X) after achlorine dioxide bleaching stage requires a sodium hydroxide charge ofabout 0.2% (w/w) relative to the mass of the pulp. The pulp bleachingsequence produces a pulp having a kappa number of about 5.7.

iv) ECOPULP TX-1200C xylanase at a pH of about 7.2 (also Do-X) after achlorine dioxide bleaching stage requires a sodium hydroxide charge ofabout 0.3% (w/w) relative to the mass of the pulp. The pulp bleachingsequence produces a pulp having a kappa number of about 5.5.

The results shown in Table 2 suggest that xylanase treatment of pulpafter a chlorine dioxide bleaching stage in accordance with the methodof the present invention reduces the amount of base, such as, but notlimited to NaOH which is required to adjust the pH of the pulp tobetween about 9 and about 12, which is typical of most alkalineextraction stages. Further, Table 2 suggests that a mild extractionstage with xylanase treatment may replace an alkaline extraction stageand yield a pulp that is brighter than a pulp which is treated in analkaline extraction stage in the absence of xylanase. Such anenzyme-treated, mildly extracted pulp which is bleached without analkaline extraction stage may result in a higher pulp strength, yield,or both, compared to a conventionally bleached pulp.

The method of the present invention is also illustrated in Table 1(Example 6), which shows mild extraction stages in the presence ofxylanase, oxygen and hydrogen peroxide. These “Xop” stages canoutperform conventional Eop stages while decreasing the use of sodiumhydroxide. For example in the case presented in Table 1, the amount ofsodium hydroxide is decreased by about 50%, while achieving equivalentbleached brightness values. A saving in amount of ClO₂ is also observedwith a Xop stage.

The method of the present invention is also illustrated in Table 3(Example 9), which shows mild extraction stages in the presence ofxylanase, oxygen and hydrogen peroxide, in comparison with conventionalxylanase stages. The mild extraction stages comprising xylanase (Xop)perform as well as conventional extraction stages, however the amount ofsodium hydroxide required to achieve the same bleached brightness levelsis significantly decreased. Furthermore, there is a decrease in theamount of ClO₂ needed to achieve similar bleached brightness levels inthe use of an Xop stage.

The method of the present invention comprises treating partiallybleached pulp with a xylanase in a mild extraction stage at a pH ofabout 3 to 8. As is evident to someone of skill in the art, the pH ofthe mild extraction stage may change during the stage. Thus, the methodof the present invention also contemplates mild extraction stages whichcommence at an initial pH outside of about 3 to about 8 and which finishat a pH inside a pH range of about 3 to about 8.

The above description is not intended to limit the claimed invention inany manner. Furthermore, the discussed combination of features might notbe absolutely necessary for the inventive solution.

The present invention will be further illustrated in the followingexamples. However, it is to be understood that these examples are forillustrative purposes only, and should not be used to limit the scope ofthe present invention in any manner.

EXAMPLE 1 Determination of Kappa Number

The kappa number of the pulp is determined using the protocol describedin: TAPPI method for Kappa number of pulp (T 236 cm-85) from TAPPI TestMethods 1996-1997, which is herein incorporated by reference. Briefly,the kappa number is the volume (in milliliters) of a 0.1 N potassiumpermanganate solution consumed by one gram of moisture-free pulp underthe conditions specified in the method. The results are corrected to 50%consumption of the permanganate added.

The kappa number determination is performed at a constant temperature of25° C.±0.2° C. with continuous agitation. However, it is possible tocorrect for variations in temperature as is described below.

The moisture content of the pulp is determined in accordance with TAPPIT 210 “Sampling and Testing Wood Pulp Shipments for Moisture” which isherein incorporated by reference. Briefly, the pulp specimen isdisintegrated in about 800 mL of distilled water and stirred. 100 mL of0.1 N potassium permanganate and 100 mL of 4N sulfuric acid (bringingthe total volume to about 1 L) are added to the slurry and allowed toreact for 10 minutes. At the end of the 10 minute period, the reactionis stopped by adding 20 mL of 1.0 N potassium iodide and the solution istitrated with 0.2 N sodium thiosulfate.

The kappa number of the pulp may be calculated using the followingformula:K=(p×f)/wwherein:p=(b−a)N/0.1,and wherein;

-   K is the kappa number;-   f is the factor for correction to a 50% permanganate consumption,    depending on the value of p (f=w*10^((0.00093×(p−50))));-   w is the weight in grams of moisture-free pulp in the specimen;-   p is the amount of 0.1 N potassium permanganate solution consumed by    the test specimen in mL;-   b is the amount of the thiosulfate solution consumed in a blank    determination in mL;-   a is the amount of thiosulfate solution consumed by the test    specimen in mL; and-   N=normality of the thiosulfate solution.

Correction of the kappa number of the pulp for determinations made attemperatures between 20° C. and 30° C. may be made using the formula:K=p×f(1+0.013(25−t))/wwherein t is the actual reaction temperature in degrees Celsius, and p,f, and w are as defined above.

EXAMPLE 2 Standard Assay for the Measurement of Xylanase Activity

Xylanase Assay #1:

The endo xylanase assay is specific for endo-1,4-beta-D-xylanaseactivity. On incubation of azo-xylan (oat) with xylanase, the substrateis depolymerized to produce low-molecular weight dyed fragments whichremain in solution on addition of ethanol to the reaction mixture. Highmolecular weight material is removed by centrifugation, and the colourof the supernatant is measured. Xylanase activity in the assay solutionis determined by reference to a standard curve.

Substrate: The substrate is purified (to remove starch and beta-glucan).The polysaccharide is dyed with Remazolbrilliant Blue R to an extent ofabout one dye molecule per 30 sugar residues. The powdered substrate isdissolved in water and sodium acetate buffer and the pH is adjusted to4.5.

Assay: Xylanase is diluted in 0.5M acetate buffer at pH 4.5. Twomillilitres of the solution is heated at 40° C. for 5 minutes. 0.25 mLof pre-heated azo-xylan is added to the enzyme solution. The mixture isincubated for 10 minutes. The reaction is terminated and high molecularweight substrate is precipitated by adding 1.0 mL of ethanol (95% v/v)with vigorous stirring for 10 seconds on a vortex mixer. The reactiontubes are allowed to equilibrate to room temperature for 10 minutes andare then centrifuged at 2000 rpm for 6-10 minutes. The supernatantsolution is transferred to a spectrophotometer cuvette and theabsorbance of blank and reaction solutions measured at 590 nm. Activityis determined by reference to a standard curve. Blanks are prepared byadding ethanol to the substrate before the addition of enzyme.

The following assay may also be used to quantify xylanase activity.

Xylanase Assay #2:

The quantitative assay determines the number of reducing sugar endsgenerated from soluble xylan. The substrate for this assay is thefraction of birchwood xylan which dissolves in water from a 5%suspension of birchwood xylan (Sigma Chemical Co.). After removing theinsoluble fraction, the supernatant is freeze dried and stored in adessicator. The measurement of specific activity is performed asfollows: Reaction mixtures containing 100 μL of 30 mg/mL xylanpreviously diluted in assay buffer (50 mM sodium citrate, pH 5.5 or thepH optimum of the tested xylanase), 150 μL assay buffer, and 50 μL ofenzyme diluted in assay buffer were incubated at 40° C. (or thetemperature optimum of the tested xylanase). At various time intervals50 μL portions are removed and the reaction is stopped by diluting in 1mL of 5 mM NaOH. The amount of reducing sugars is determined using thehydroxybenzoic acid hydrazide reagent (HBAH) (Lever, 1972, AnalyticalBiochem 47: 273-279). A unit of enzyme activity is defined as thatamount generating 1 μmol reducing sugar in 1 minute at 40° C. (or at theoptimum pH and temperature of the enzyme).

EXAMPLE 3 Preparation of Chlorine Dioxide

Chlorine dioxide was made in the lab by the standard procedure ofpassing a mixture of chlorine gas and nitrogen through a series ofcolumns containing sodium chlorite, and collecting the evolved gas incold water. The chlorine dioxide was stored refrigerated at aconcentration of 10.4 grams per litre in water. Further detailsregarding the preparation of chlorine dioxide may be found in ChlorineDioxide Generation published by Paprican, Pointe Claire, Quebec (whichis herein incorporated by reference).

EXAMPLE 4 Conventional Xylanase Treatment of Pulp

This is the procedure for carrying out conventional xylanase treatment,prior to bleaching of the pulp in a Do stage. A 15 g pulp sample havinga predetermined kappa number is adjusted to a consistency of 10%(wt/vol) with deionized water and the pH of the pulp is adjusted between6.8 and 7 with a 10% solution of Na₂CO₃. The pulp sample is heated to57° C. prior to adding TrX-HML-AHAE xylanase, which is commerciallyavailable from Iogen Corporation. Enzyme is added to samples and thepulp samples are incubated at 57° C. for 60 minutes. Following theincubation period the reaction is stopped by lowering the pH to between2.5 and 3 by the addition of hydrochloric acid and by cooling thesamples in an ice water bath.

The enzyme dose is 0.5 to 2.0 units of xylanase activity (measuredaccording to the first xylanase assay described in Example 2) per gramof pulp. For comparative purposes, pulp samples may be mock-treatedunder conditions which lack xylanase to facilitate comparison of thedifferent bleaching sequences.

EXAMPLE 5 Chlorine Dioxide Bleaching of Hardwood Pulp Samples

Pulp samples are subjected to chlorine dioxide bleaching stages whichare similar to those described in Glossary of Bleaching Terms CPPAtechnical section, which is herein incorporated by reference, describingoptimum conditions of 1.0%-2.3% ClO₂ on pulp, 40-60° C., 3-10% pulpconsistency, 30-60 minute incubation period, pH 2.5 to 3.0.

Chlorine Dioxide Bleaching (Do) Stage

The first chlorine dioxide bleaching stage is the Do stage. The pulpmixture is cooled to 4° C. to minimize evaporation prior to chlorinedioxide addition. ClO₂ is added to the pulp and the system is maintainedin a heat-sealable plastic bag. Pulp kappa factors of 0.15, 0.17, 0.19and 0.21 are used to calculate the chlorine dioxide charge required inthe bleaching stage. The chlorine dioxide charge may be determined usingthe following formula:Chlorine dioxide charge (kg/ton pulp)=10×kappa factor×kappa number/2.63

Based on a kappa factor of 0.17 and a pulp kappa number of 13.9, thecorresponding chlorine dioxide usage is 9 kg/ton pulp. After ClO₂addition, the pulp comprises 4% consistency, pH 2.5 to 3.0. The bags areplaced in a 50° C. water bath for 60 minutes. Following the incubationperiod, pulp samples are washed with 2L of tap water. Subsequently, Dopulp samples are subjected to a mild extraction with xylanase (X), orsubjected to a conventional alkaline extraction stage (Eop).

Mild Extraction (Xop)

Xylanase treatments in mild extraction Xop stages are carried out asdescribed in Example 4, with the following changes. The pulp samples aresubjected to a mild extraction stage (Xop) after the first chlorinedioxide bleaching stage (Do). The Xop stage comprises incubating pulpsamples at 60° C., 10% (wt/vol) consistency, with a sodium hydroxidecharge of 0.2-0.4%, a hydrogen peroxide charge of 0.3% (wt/wt) and anoxygen pressure of 5 psig, to consume 6 kg oxygen per tonne pulp, for 60minutes. The pH of the extraction medium is about 7.5 at the end of theincubation. Following the incubation period, each pulp sample is washedwith 2L of tap water.

Alkaline Extraction Stage (Eop)

After the first chlorine dioxide bleaching stage (Do), the control pulpsamples are subjected to an alkaline extraction stage (Eop). The Eopstage comprises incubating pulp samples at 75° C., 10% (wt/vol)consistency, with a sodium hydroxide charge of 1.2%, a hydrogen peroxidecharge of 0.3% (wt/wt) and an oxygen pressure of 5 psig, to consume 6 kgoxygen per tonne pulp, for 60 minutes. The pH of the extraction mediumis about 11.5 at the end of the incubation. Following the incubationperiod, each pulp sample is washed with 2L of tap water.

Chlorine Dioxide Bleaching (D₁) Stage

Regardless of whether the extraction stage is mild or conventional, allpulps are subjected to similar D₁ stages. The D₁ stage is performed in asimilar manner to the Do stage. Briefly, pulp samples are adjusted to aconsistency of 10% (wt/vol) and incubated at pH 3.6 to about 4, 75° C.for 180 minutes. The D₁ chlorine dioxide charges are chosen tocorrespond with kappa factors of 0.11, 0.13, 0.15 and 0.17. Followingthe incubation period, each pulp sample is washed with 2L of tap water.Following the extraction the brightness of the pulp may be measuredaccording to Example 7. This is the D₁ brightness of the pulp. The pulpmay be further bleached by performing a second extraction stage and athird chlorine dioxide bleaching stage.

Second Extraction Stage (E₂)

Following the D₁ stage the pulp is subjected to a second extractionstage. The extraction stage comprises incubating pulp at 75° C. for 90minutes with a 1% NaOH charge. The pulp consistency is 10% (wt/vol) andthe pH of the pulp following the incubation period is about 11.3.Following the extraction stage, each pulp sample is washed with 2L oftap water.

Chlorine Dioxide Bleaching (D₂)Stage

The D₂ stage is performed in a similar manner to the other chlorinedioxide bleaching stages. The pulp samples are adjusted to a consistencyof 10% (wt/vol). The chlorine dioxide charge is 0.29% (w/w) for eachsample. Samples are incubated at 75° C. for a period of 180 minutes. Thefinal pH of the chlorine dioxide bleaching stage is about pH 4.

Following the Chlorine Dioxide Bleaching (D₂) stage, the brightness ofthe pulp may be measured according to Example 7. This is the D₂brightness of the pulp.

EXAMPLE 6 Mild Extraction with Xylanase to Decrease Use of SodiumHydroxide and Chlorine Dioxide

Unbleached hardwood kraft pulp (kappa number 15.6) from a mill in Quebecwas subjected to D stages as described in Example 5. Following this, thepulp was subjected to mild X extraction stages as in Example 5, using 1IU/g of TrX-HML-AHAE xylanase, from Iogen Corporation, or 1 IU/g ofActinomadura flexuosa xylanase A produced as described in U.S. Pat. No.5,935,836. Control pulps were subjected to conventional Eop stages asdescribed in Example 5. Following the mild or conventional extractions,the pulps were fully bleached using DED sequences described in Example5.

The results are shown in Table 1. The use of mild extraction stagescontaining xylanase can increase the bleached brightness of the pulp, ata wide range of bleaching chemical charges, as reflected by Total Kappafactor. The mild extraction stages also decrease the use of sodiumhydroxide and of chlorine dioxide, thereby offering the mill thepossibility of decreasing bleaching chemical costs.

TABLE 1 Bleaching with mild extraction stages TrX-HML- Conventional AHAEActinomadura extraction xylanase flexuosa xylanase- Parameter DoEopDEDDoXopDED A DoXopDED Bleached brightness TKf 0.35 85.6 85.8 86.3 TKf 0.4085.8 86.7 86.8 TKf 0.45 86.8 87.4 87.6 TKf 0.50 87.7 87.5 87.8 NaOH inExtraction, 10.0 4.3 4.3 Kg/t ClO2 to 87 Brightness, 23.3 21.2 20.4 Kg/t

EXAMPLE 7 Measuring Pulp Brightness

Pulp brightness is measured according to the method disclosed byPAPTAC-Standard Testing Methods July, 1997 (Standard E1 Brightness ofPulp, Paper and Paperboard, which is herein incorporated by reference).

Briefly, a 3.75 g sample of pulp is used to form a brightness pad. Apulp sample is. placed in a 500 mL container and water is added to about200 mL. About 2 mL of sulfuric acid solution is added to each jar andthe contents mixed well. A pad is formed by pouring the pulp into afunnel under vacuum and subsequently pressing the pad with a plunger.Each pad is pressed between blotters using a hydraulic press. The pulppad is allowed to dry overnight at room temperature.

ISO Brightness Determination

Brightness is measured using an Elrephometer. The sample is diffuselyilluminated using a highly reflecting, integrated sphere. Reflectedlight is measured at right angles to the sample. Reflectance is comparedto absolute reflectance based on a perfectly reflecting, perfectlydiffusing surface which is considered to exhibit a brightness of 100%.Magnesium oxide is one standard which is used to compare to pulpbrightness. A blue light of wavelength 457 nm is used for the brightnessreading.

EXAMPLE 8 Mild Extraction with Xylanase to Decrease Use of SodiumHydroxide

Unbleached hardwood pulp having a kappa number of 14.9 was obtained froma mill in Quebec. The pulp is washed with water and adjusted to a pHbetween 2.5 to 3.0 using HCl. Several 10 gram samples of pulp aresubjected to a chlorine dioxide (Do) bleaching stage according to theGlossary of Bleaching Terms of the CPPA technical section, which isherein incorporated by reference describing optimum conditions of1.0%-2.3% ClO₂ on pulp, 3-10% pulp consistency, 30-60 minute incubationperiod, pH 2.5-3.0. Briefly, ClO₂ is added to the pulp and the system ismaintained in a heat-sealable plastic bag. The pulp mixture is cooled to4° C. to minimize evaporation before ClO₂ addition. The kappa factor isrecommended to be about 0.17 to avoid formation of furans and dioxins(Glossary of Bleaching Terms of CPPA Technical Section, which isincorporated herein by reference). The chlorine dioxide charge may beestimated using the formulas in Example 5.

Based on a kappa factor of 0.17, the corresponding chlorine dioxidecharge is 9.6 kg/ton pulp. After ClO₂ addition, the pulp comprises 4%consistency. The bags are placed in a 50° C. water bath for 60 minutes.

After the D stage, the pulp is washed with tap water over a vacuumfunnel. The pulp is adjusted to 10% consistency with deionized water.For enzyme-treated pulps, a mild extraction was carried out as describedin Example 5, with the following exceptions.

For the first enzyme-treated pulp, the initial pH of the pulp isadjusted to 6.7 with sodium hydroxide. The pulp is heated to 60° C. anda xylanase enzyme, TrX-HML-AHAE, available commercially from IogenCorporation, is added to the pulp. The enzyme dosage is 0.7 units pergram of pulp. The pulp bag is placed in a 60° C. water bath for 1 hour.

For the second enzyme-treated pulp, the initial pH of the pulp isadjusted to 7.4 with sodium hydroxide. The pulp is heated to 60° C. anda xylanase enzyme, HTX18, available commercially from Iogen Corporation,is added to the pulp. The enzyme dosage is 0.8 units per gram of pulp.The pulp bag is placed in a 60° C. water bath for 1 hour.

For the third enzyme-treated pulp, the initial pH of the pulp isadjusted to pH 7.4 with sodium hydroxide. The pulp is heated to 60° C.and xylanase from Actinomadura flexuosa xylanase A, produced asdescribed in U.S. Pat. No. 5,935,836 is added to the pulp. The enzymedose is 0.7 units per gram of pulp. The pulp bags are placed in a 60° C.water bath for 1 hour.

For the fourth enzyme-treated pulp, the initial pH of the pulp isadjusted to 6.7 with sodium hydroxide. The pulp is heated to 60° C. andxylanase from Actinomadura flexuosa xylanase A, produced as described inU.S. Pat. No. 5,935,836 is added to the pulp. The enzyme dosage is 0.7units per gram of pulp. The pulp bags are placed in a 60° C. water bathfor 1 hour.

For the untreated control pulps, conventional extraction was carried outas described in Example 5, with the following exceptions. The initial pHof the pulp is adjusted to 11.4 with sodium hydroxide. The pulp isheated to 60° C., and the pulp bags are placed in a 60° C. water bathfor 1 hour.

After incubation, all pulps are subsequently washed with tap water, andthe kappa number of the pulp is determined. The results are shown inTable 2.

TABLE 2 Bleaching using a Conventional extraction stage (E), or a mildextraction stage with xylanase (X) Bleaching Enzyme in mild Kappa NaOH(% w/w sequence extraction Number on pulp) DoE Untreated control, 6.31.2 pH 11.2 extraction DoX TrX-HML- 5.4 0.2 AHAE, pH 6.8 DoX HTX-18, pH7.2 6.1 0.2 DoX Actinomadura 5.7 0.2 flexuosa xylanase A, pH 6.8 DoXActinomadura 5.5 0.3 flexuosa xylanase A, pH 7.2

These results show that it is possible to use mild extraction stageswith xylanase enzymes and decrease the use of sodium hydroxide. Thekappa number of the pulp after the mild extraction stage with xylanaseis lower than the kappa number after the conventional extraction stage.This indicates that a further savings in chlorine dioxide is possible.

EXAMPLE 9 Mild Extraction with Xylanase-Comparison with Conventional andDouble Xylanase Stages

Unbleached hardwood kraft pulp (kappa number 15.6) from a mill in Quebecwas subjected to a conventional enzyme treatment (X) stage as describedin Example 4, using 0, 0.5, or 1 iu/g of TrX-HML-AHAE at pH 6.5, 55° C.,for 1 hour. This pulp was then subjected to Do stages as described inExample 5. Following this, the pulp was subjected to mild Xop extractionstages using 0, 0.5, or 1 IU/g of TrX-HML-AHAE xylanase, from IogenCorporation, as described in Example 5. Control pulps were subjected toconventional D and E stages without xylanase treatment as described inExample 5. Following the mild or conventional extractions, the pulpswere fully bleached using DED sequences described in Example 5.

The results are shown in Table 3. The use of mild extraction stagescontaining xylanase can increase the bleached brightness of the pulp, ata wide range of bleaching chemical charges, as reflected by Total Kappafactor. The mild extraction stages also decrease the use of sodiumhydroxide and chlorine dioxide, thereby offering the mill thepossibility of decreasing bleaching chemical costs.

TABLE 3 Bleaching with mild extraction stages Conventional Trx-HML-TrX-HML- TRX-HML- Conventional AHAE AHAE AHAE extraction xylanasexylanase EB xylanase Parameter DoEopDED DoXopDED XDoEopDED XDoXopDEDXylanase on brownstock, 0.0 0.0 1.0 0.5 (iu/g) Xylanase in Xop (IU/g)0.0 1.0 0.0 0.5 Total Xylanase (IU/g) 0.0 1.0 1.0 1.0 Final pH of Eop orXop 11.5 7.4 11.5 7.4 Temperature (° C.) of Eop 75 60 75 60 or XopBleached brightness TKf 0.35* 85.6 85.8 85.9 86.1 TKf 0.40 85.8 86.786.8 87.2 TKf 0.45 86.8 87.4 87.7 88.3 TKf 0.50 87.7 87.5 87.9 88.9 NaOHin Extraction (Kg/t) 10.0 4.3 10.0 4.3 ClO2 to 87 Brightness, 23.3 21.220.9 18.9 Kg/t) *Total kappa factor (TKf) includes a small contributionby hydrogen peroxide.

The results shown in Table 3 indicate that single (Xop) and multiple (X,and Xop) xylanase treatments in mild extraction stages are moreeffective than bleaching sequences that do not comprise an Xop stage.Furthermore, the above results demonstrated that multiplexylanase-treatments may be more effective than a single xylanasetreatment when a fixed amount of xylanase is used for the entirebleaching process, while a similar degree of bleached brightness isobtained with less NaOH and ClO₂.

The mild extraction stage offers opportunities beyond that of aconventional xylanase treatment stage. The mild extraction stage, withxylanase, has similar bleached brightness as the conventional xylanasestage. However, the mild xylanase extraction stage also saves asignificant amount of sodium hydroxide for pH adjustment.

The combination of a conventional xylanase treatment and a mildextraction stage is especially beneficial to the mill. The benefitsinclude a higher degree of chlorine dioxide savings and also sodiumhydroxide savings, with the same xylanase dosage as with conventional ormild extraction treatment on its own.

All references are herein incorporated by reference.

The present invention has been described with regard to preferredembodiments. However, it will be obvious to persons skilled in the artthat a number of variations and modifications can be made withoutdeparting from the scope of the invention as described herein.

REFERENCES

-   Ericksson, K. E. L., (1990) Wood Science and Technology 24; 79-101.-   Lüthi, E., Jasmat, N. B., and Bergquist, P. L. (1990) Appl. Environ.    Microbiol. 56:2677-2683.-   Paice, M. G., R. Bernier, and L. Jurasek, (1988) Biotechnol. and    Bioeng. 32,235-239.-   Pommier, J. C., J. L. Fuentes, and G. Goma, (1989) Tappi Journal,    187-191.-   Reeve and Dence (1996) Pulp Bleaching Principles and Practice. Tappi    Press, Atlanta, Ga.-   Simpson, H. D., Haufler, U. R., and Daniel, R. M. (1991)    Biochem. J. (1991) 277:413-417.-   Winterhalter C. and Liebl, W. (1995) Appl. Environ. Microbiol.    61:1810-1815.

1. A method of bleaching chemical pulp comprising the steps of: a) treating chemical pulp in a first chlorine dioxide bleaching stage and then washing the pulp with a first water wash to produce a partially bleached pulp; b) treating said partially bleached pulp with a xylanase directly subsequent to said first chlorine dioxide bleaching stage and said first water wash to hydrolyze xylan present in said partially bleached pulp, and then washing the pulp with a second water wash to produce an enzymatically treated pulp, wherein said treating is carried out at a pH of between about 3 and about 8; and c) treating said enzymatically treated pulp to a second chlorine dioxide bleaching stage directly subsequent to the xylanase treatment stage and said second water wash (step b).
 2. The method of claim 1, wherein said chemical pulp comprises kraft pulp, soda pulp or sulfite pulp.
 3. The method of claim 1, wherein at least one of said first and second chlorine dioxide bleaching stages comprises chlorine dioxide and chlorine.
 4. The method of claim 1, further comprising an oxygen delignification stage, a xylanase treatment stage, or a combination thereof, prior to said step of treating chemical pulp in the first chlorine dioxide bleaching stage (step a). 